Ethylene oxide-capped polyols have long been valuable in the polyurethane industry because of the favorable reactivity of their primary hydroxyl groups with polyisocyanates. EO-capped polyols are normally made in two steps. First, propylene oxide (or a mixture of propylene oxide and ethylene oxide) is polymerized in the presence of a basic catalyst (usually potassium hydroxide) to produce a polyol that contains mostly or exclusively secondary hydroxyl groups. Second, ethylene oxide is added to the catalyst-containing mixture to convert some or most of the secondary hydroxyl groups to primary hydroxyl groups. The process conveniently uses the same catalyst (usually KOH) for both the propoxylation and ethoxylation steps.
Double metal cyanide (DMC) catalysts such as zinc hexacyanocobaltate offer many advantages for making polyether polyols. Polyols with exceptionally low unsaturation levels compared with KOH polyols can be made. The advantages of low-unsaturation polyols for making polyurethanes with improved properties are well documented. DMC catalysts also have exceptional activity, so polyol production is efficient. The best DMC catalysts now known are active enough to be used at such low concentrations that back-end purification of the polyols is often unnecessary.
In spite of the many advantages of DMC catalysts for making polyether polyols, an important drawback remains: ethylene oxide-capped polyols cannot be made directly using a DMC catalyst. In other words, one cannot prepare an oxypropylene polyol by DMC catalysis, and then simply add EO to cap the polyol (as is done with KOH). When EO is added to an polyoxypropylene polyol made with a DMC catalyst, the resulting product is a heterogeneous mixture of: (1) mostly unreacted polyoxypropylene polyol; and (2) a minor proportion of highly ethoxylated polyoxypropylene polyol and/or polyethylene oxide.
The usual way to make an EO-capped polyol from a DMC-catalyzed polyol involves recatalysis. See, for example, U.S. Pat. Nos. 4,355,188 and 4,721,818. First, an oxypropylene polyol (or random EO-PO copolymer) is made by DMC catalysis. Second, a basic catalyst such as an alkali metal, alkali metal hydride, alkali metal alkoxide, alkali metal hydroxide, or the like, is added. The basic catalyst deactivates the DMC catalyst. Typically, the polyol must then be stripped to remove water or alcohol introduced (even in trace amounts) with the basic catalyst. Finally, ethylene oxide is added to cap the polyol with oxyethylene units.
The recatalysis approach has some important disadvantages. First, many basic catalysts (particularly the alkali metals and alkali metal hydrides) are highly reactive, moisture-sensitive, and difficult to handle safely. Second, a dedicated reactor is needed for base-catalyzed EO capping because DMC catalysts are poisoned by even trace amounts of residual base. Thus, a reactor used for base-catalyzed EO capping must be cleaned scrupulously before another DMC-catalyzed reaction can be performed in the same reactor. On a large scale, this is impractical, so a dedicated reactor just for EO-capping is needed. Third, stripping polyols to remove water or alcohol is time consuming, energy intensive, and often gives sporadic results. Incomplete stripping gives hazy polyols that contain polyethylene oxide (reaction product of traces of water or alcohol with ethylene oxide), and hazy polyols are commercially undesirable.
An improved process for making EO-capped polyols from DMC-catalyzed polyols is needed. Preferably, the process would overcome the need for recatalysis. It would avoid highly reactive, moisture-sensitive ethoxylation catalysts, and would eliminate the need to strip water or alcohols from the polyol intermediate prior to addition of ethylene oxide. A preferred process would not require a dedicated reactor just for EO-capping. Ideally, the process would be easy to practice, and would provide a way to make low unsaturation polyols with high primary hydroxyl group contents.